There is interest, from an environmental point of view, to reduce the discharge of environmental pollutants from engine-driven vehicles as much as possible. In consideration of this, hybrid vehicles have been developed which utilize an electric motor in addition to an internal combustion engine to drive a driving wheel or driving wheels of the vehicle using the electric motor.
Hybrid vehicles achieve reductions in noise and air pollution by using mainly the electric motor as the power source for the vehicle while the vehicle operates steadily. Hybrid vehicles additionally use the engine to avoid the drawbacks of electric vehicles driven solely by an electric motor. For example, the additional use of the engine avoids problems such as the limited running distance per battery charge, and the inadequate response during rapid start-up, high-load or high-speed operating conditions due small power generation output from the electric motor.
Hybrid vehicles include parallel hybrid vehicles, in which at least one of an internal combustion engine and an electric motor can be switched on and off depending on the running condition of the vehicle and the remaining amount of electricity in a battery (e.g., secondary battery) charged by an electric generator. Another type of the hybrid vehicles is series hybrid vehicles, in which a driving wheel of the vehicle is driven by a drive motor, which in turn is driven solely by electricity generated by an electric generator that is driven by an internal combustion engine.
Series-parallel hybrid vehicles have also been developed, a combination of the series hybrid and the parallel hybrid vehicles, in which engine output is distributed by a power distribution device using a planetary gear mechanism to drive a driving wheel, as disclosed for example in Japanese Patent No. JP 2003-191761.
The power distribution device splits engine power into a vehicle driving force to be mechanically transmitted to the driving wheel to drive the driving wheel directly, and an electricity generation driving force to actuate the electric generator to generate electricity. That is, the power distribution device uses a portion of engine power to rotate the driving wheel and another portion to drive the electric generator. The electricity generated by the electric generator is supplied to the electric motor to run the motor, and the power produced by the motor in response to the supplied electricity is added, by the power distribution device, to one of the split portions to assist the driving force communicated to the driving wheel.
The use of a hybrid drive unit using the power distribution device as described above allows the hybrid vehicle to operate the engine at the most preferable fuel consumption rate.
In general, hybrid vehicles having the power distribution device described above can have an operating state in which the vehicle is powered and propelled by both the electric motor and the internal combustion engine at the same time, as well as an operating state where the engine is stopped while the vehicle is powered by the engine and the motor. Also, hybrid vehicles having the power distribution device described above can be in an operating state where combustion does not take place in the engine, though the crankshaft of the engine rotates and the pistons coupled to the crankshaft reciprocate.
During a transitions between operating states, such as the operating states discussed above, an abrupt change in the driving wheel propulsion force causes an impact on the vehicle before the crankshaft comes to a complete stop, even when the motor is outputting constant torque.
In order to cope with such an impact, hybrid vehicles having the power distribution device use, for example, a variable valve mechanism to control pumping loss of the engine. Even with the use of the variable valve mechanism, however, the in-cylinder pressure cannot be reduced through the entire compression stroke of the engine because of limitations on the phase range within which valve timing can be varied. Thus, even with the use of the variable valve mechanism, the in-cylinder pressure remains slightly compressed, which as pumping loss causes an impact during transition to an operating state where the engine is completely stopped, that is, to a state where rotation of the crankshaft is completely stopped.
In order to cope with such an impact related to engine stop, it is also possible to use a valve stop mechanism to control the operation of valves, such as keeping intake and exhaust valves closed throughout the stroke and restrict ventilation inside the cylinder. One such valve stop mechanism is a REV (revolution-modulated valve control) system.
In this way, air compressed inside the cylinder is expanded generally equally, which reduces the crank torque necessary to maintain the rotation of the crankshaft, as compared to the case where ventilation is not restricted. That is, in general-purpose engines, a sudden change in torque can be lessened to a negligible level during transition from engine operation with ignition to an operating state without combustion but with rotation of the crankshaft and reciprocation of the pistons. With this construction, however, in the transition process to stop the rotation of the crankshaft, the compressed in-cylinder pressure restrains the rotation of the crankshaft, which causes a sudden change in torque and hence impact before the crankshaft stops.
Such impact at a transition of operating states, such as engine stop, does not affect operation by an operator in the case of automobiles, in which the engine is mounted in an engine compartment.
In recent years, the drive unit having a power distribution device, as described above, has also been applied to motorcycles.
The operating direction of a motorcycle is determined by an increase and decrease in the driving wheel propulsion force that occurs while the vehicle is turning, based on the principle of two-wheel operation. Thus, it is necessary to differentiate an increase and decrease in the driving wheel propulsion force intended by the operator and those not intended by the operator. The unintentional increase and decrease are preferably as small as possible since they can affect the operating state interpreted or sensed by the operator.
That is, in the case where the hybrid drive unit using the power transmission device disclosed in JP 2003-191761 is mounted on a motorcycle, an impact which occurs when the engine is stopped while the vehicle is running or during transition to an operating state without combustion but with continuing rotation of the crankshaft and reciprocation of the piston(s) is an unintentional increase and decrease in propulsion force that need to be decreased. This is because even an impact when the engine is stopped, which does not affect automobiles, can be sensed by operators of motorcycles as an unintentional increase and decrease in propulsion force.
In order to eliminate such impact, it is conceivable to increase the capacity of the battery to supply current to the motor or increase torque generated by the motor. However, a motorcycle has a limited mounting space compared to an automobile and thus cannot accommodate a battery which becomes larger as its charging capacity increases, or a larger motor for producing increased torque.
Thus, there is a need for a hybrid motorcycle in which impact due to fluctuations in crankshaft torque, which changes abruptly at engine stop, can be reduced so that impact during transition to an operating state related to engine stop can be reduced.